1. Field of the Invention
While the invention is subject to a number of different applications, it is especially suited for use in the transmission of wideband information over a multiplicity of narrowband channels and will be particularly described in that connection.
It is desirable when combining narrowband channels that the individual channels are free of frequency translation errors. This invention uses the transmission of two tones having a known frequency ratio as a basis for frequency and phase correction. The two tones are combined with at least a portion of the signal prior to transmission over one of the narrowband channels and means are provided at the receive end of the circuit for sensing any error in the ratio of the tone frequencies and additional means are provided for correcting any error in the ratio. At least one of the tones is also combined with signal components for transmission over the other narrowband channels so that all channels can be frequency corrected.
Overlap components are transmitted over two of the narrowband transmission circuits and one set of overlap components are automatically phase controlled so that the frequency components from one narrowband circuit matches corresponding components from a second narrowband component. In order to avoid holes in the spectrum response, overlap components should be provided for those points of the spectrum where the holes may be created. For example, where wideband intelligence is split into two segments the highest frequency edge of the lower frequency segment and the lowest frequency edge of the highest frequency segment require overlap components. Memory circuitry is also provided for maintaining the relative phase of the overlap components to a desired value during periods when the overlap components are weak or missing. The automatic frequency correction and phase correction method disclosed herein, utilizing two tones having a known frequency ratio, may also be used for SSB transmission and other systems where only one transmission channel is used.
2. Description of the Prior Art
There are a large number of narrowband voice grade circuits available at relatively low cost. However, in many situations such as the transmission of music, the intelligence transmitted requires wider bandwidth circuits. While wideband bandwidth circuits can be made available they are relatively expensive.
In U.S. Pat. No. 3,696,298 issued to Kahn and Gordon on Oct. 3, 1972, and U.S. Pat. No. 3,684,838 issued to Kahn on Aug. 15, 1972, systems were disclosed for improving the overall response of narrowband transmission links. U.S. Pat. No. 3,696,298 describes a method whereby two or more narrowband channels are combined so that relatively wideband signals may be transmitted. Use is made of frequency translation of various components so as to make those signal components suitable for transmission over a narrowband channel. Equipment utilizing this invention has been used in a number of circuits. In that equipment, however, while the resulting sound is superior to that of a narrowband circuit, there are frequency cuts or holes in the frequency response which may degrade certain types of transmission and which are apparent when one measures the overall frequency response of the circuit. For example, data signals may have large amplitude components at the cut frequency, and thus serious degradation of transmission may result.
U.S. Pat. No. 3,684,838 only requires a single narrowband channel. A small segment of that circuit is used for transmission of frequency components that will normally fall outside of the response of the narrowband circuit. For the natural sounding transmission of voice, it is desirable that sound components between 100 Hz and 250 Hz be transmitted. These components; however, do not normally pass through a narrowband telephone circuit. U.S. Pat. No. 3,684,838 discloses a system for transmitting such components along with the bulk of the high energy components of the voice. While this system greatly improves voice transmission it does not improve higher frequency response.
Other inventions that are related to segmented signal waves or to phasing of signal components are my U.S. Pat. No. 3,528,012 issued Sept. 8, 1970 and U.S. Pat. No. 3,364,428 issued Jan. 16, 1968.